How to use the circle calculator
Choose what you measured
Select Radius if you measured from the centre to the edge, or Diameter if you measured straight across the circle through its centre. Enter one positive value in the chosen unit and select Calculate. Changing the type or value clears the old result.
Radius and diameter
The diameter is twice the radius: d = 2r. If diameter is the known measurement, radius is d ÷ 2. These two lengths use the original unit.
Circumference: distance around the circle
Circumference is C = 2πr, which is also C = πd. Use it for the edge of a circular table, wheel, lid, or any other complete circle.
Area: surface inside the circle
Area is A = πr² and uses square units. The radius must be squared, so using the diameter directly in the area formula would make the result four times too large.
Worked examples
Radius of 5 cm
Diameter = 10 cm, circumference = 10π ≈ 31.42 cm, and area = 25π ≈ 78.54 cm².
Diameter of 12 m
Radius = 6 m, circumference = 12π ≈ 37.7 m, and area = 36π ≈ 113.1 m².
Limits and error checks
Enter one positive finite number. Do not include unit symbols, fractions, formulas, or thousands separators. The displayed decimal is rounded, while the calculation uses JavaScript’s full numeric precision. This tool covers complete circles only—not arcs, sectors, polygons, or spheres.
Frequently asked questions
Should I choose radius or diameter?
Choose the measurement you already know. Radius runs from the centre to the edge; diameter runs edge to edge through the centre.
Why does area use square units?
Area measures a surface and multiplies the radius by itself, so centimetres become cm² and metres become m².
Does changing the unit convert my value?
No. The selector labels the input and output. Enter the measurement in the unit you select.
Why did the result disappear after editing?
The old answer no longer matches the current measurement, so it stays clear until you calculate again.